1. Field of the Invention
The present invention relates to an aluminum-containing metal composite material and a process for producing the same. More particularly, the present invention relates to an aluminum-containing metal composite material having a satisfactory hydrophilic property and water-resistance and usable for heat-exchangers, for example, evaporators for car air-conditioners, and a process for producing the same with a high efficiency.
2. Description of the Related Art
It is well known that a conventional heat-exchanger has a plurality of tubes through which a first heat-conductive fluid flows and a plurality of fins extending from the tubes and being exposed to a second heat-conductive fluid. Generally, the larger the total surface area through which heat is exchanged between the first and second heat-conductive fluids, the higher the heat exchange efficiency. Therefore, the heat-exchanger, for example, an evaporator, is designed so that the cooling area of the evaporator is made as large as possible, to enhance the cooling effect of the evaporator. Also, to make the size of the evaporator as small as possible, the gaps between the fins is made very small.
As a result of the above-mentioned design, moisture in the air is condensed to form water drops between the fins and the water drops formed between the fins causes the flow of the second heat-conductive fluid to be hindered and the heat exchange efficiency of the heat exchanger to decrease. Also the water drops are scattered into the downstream side of the evaporator so as to reduce the heat exchange efficiency.
Further, the condensed water drops between the fins cause dust in the air to adhere to the fins and to be accumulated in the gaps between the fins. The adhered dust causes a propagation of bacteria in the gaps between the fins, and the propagated bacteria produce metabolic products which generate an unpleasant odor.
Japanese Unexamined Patent Publication (Kokai) No. 61-250,495 discloses a heat exchanger in which the above-mentioned disadvantages are eliminated. In this heat exchanger, a chemical conversion layer is formed on a substrate comprising an aluminum-containing metal material and a hydrophilic resinous coating layer is formed on the chemical conversion layer. This hydrophilic resinous coating layer effectively prevents the formation of the water drops between the fins and the increase in the flow resistance of the second heat-conductive fluid due to the water drops. Also, the Japanese publication states that the generation of the unpleasant odor derived from the bacterial metabolic products can be prevented by adding an antibacterial agent or a deodorant to the resinous coating layer.
Nevertheless, the inventors of the present invention have in depth investigated the technique of the Japanese publication and found that this technique is disadvantageous in that the hydrophilic resinous coating layer is gradually eluted in the condensed water and cannot be made to appear over a long period of employment.
Namely, due to the poor water resistance of the hydrophilic resinous coating layer, in the employment environment in which a heat exchange surface of, for example, an evaporator, is always brought into contact with water, the hydrophilic resinous coating layer is consumed to an extent that during a practical use for about one year, the amount of the hydrophilic resinous coating layer decreases to about 10% of the initial amount thereof, and the resultant coating layer exhibits a significantly reduced hydrophilic property and antibacterial property. Also, the inventors have found that as a result of the elution of the resinous coating layer, the surface of the aluminum-containing metal substrate partially exposed to the outside and slightly corroded. This corrosion causes a stimulative odor to be generated.
As an attempt to prevent the elution of the hydrophilic resinous coating layer in the condensed water, Japanese Unexamined Patent Publication (Kokai) No. 1-270,977 discloses a process for coating an aluminum surface with a hydrophilic resinous layer by applying a mixture solution of a water-soluble, cross-linkable acrylamide polymer (P.sub.1), a water-soluble polymer (P.sub.2) having hydrophilic groups, for example, carboxyl, sulfonic or phosphoric groups, amino groups or quaternary ammonium groups, and a water-soluble cross-linking agent compatible with the polymers (P.sub.1) and (P.sub.2) to an aluminum surface and drying the coated mixture solution layer.
Also, as another attempt, Japanese Unexamined Patent Publication (Kokai) No. 3-26,381 discloses a process for coating an aluminum surface with a hydrophilic resinous coating layer by treating the aluminum surface with a mixture solution of a water-soluble polyvinyl alcohol and/or derivative thereof (P.sub.1), a water-soluble polymer (P.sub.2) having carboxylic, sulfonic or phosphoric groups and a water-soluble cross-linking agent compatible with the polymers (P.sub.1) and (P.sub.2).
In these prior art processes, the water-soluble polymers (P.sub.1) and (P.sub.2) are cross-linked and made water-insoluble. The resultant resinous layers are difficult to dissolve in the condensed water. When the resultant aluminum material having the cross-linked resinous coating layer is used in the formation of an air-conditioner, it is alternately wetted with the condensed water and dried. In the wetting-drying cycles, the resinous coating layer is alternately swollen with water and dried. The wetting-drying cycles cause the resinous coating layer to be deteriorated and then broken and removed.
Usually, where an air conditioner having complicated heat-exchange surfaces is coated with the resinous solution by immersion, it is difficult to uniformly distribute the resinous solution on the complicated surfaces of the air conditioner. Namely, in some portions of the air conditioner, the resinous solution is distributed in an excessive amount. The deterioration of the resinous coating layer significantly occurs in the excessively coated portions. The removed resinous layer are scattered throughout the air conditioner when it is operated. Also, the removal of the resinous coating layer causes portions of the aluminum surface to be exposed to the outside, and a stimulative odor to be generated due to the corrosion of the exposed surface portions. Therefore the above-mentioned prior arts are not satisfactory to provide an aluminum material having a resinous coating layer and capable of practical use over a long period without removal of the resinous coating layer.